This invention relates to magnetic induction accelerators of the betatron type and, more particularly, to active electron control circuits for betatrons.
1. Cross Reference to Related Patents
The invention of the present application is related to the invention described in the commonly-owned U.S. Pat. No. 5,077,530, issued Dec. 31, 1991 to the present inventor for Low-voltage Modulator for Circular Induction Accelerator, and to the invention described in the commonly-owned U.S. Pat. No. 5,122,662, issued Jun. 16, 1992 to the present inventor et al. for Circular Induction Accelerator in Borehole Logging, the disclosures of both of which are hereby incorporated by reference.
2. Background of the Invention
Prominent among performance criteria for betatrons, and in particular for miniature betatrons, as used, for example, in borehole logging tools, are power efficiency, beam end-point energy stability, and beam energy and flux stability under changing ambient temperature. Specifically as to power efficiency, two contributing factors may be distinguished, namely (i) the efficiency of a modulator in the conversion and delivery of power to the betatron magnet and (ii) the efficiency of conversion of the magnet excitation power into beam power. The former, namely modulator efficiency, is a primary concern of the aforementioned U.S. Pat. No. 5,077,530. The present invention is mainly concerned with the latter, namely the efficiency of magnet-excitation power conversion.
In a betatron, almost all the magnetic-circuit excitation energy is stored in the air gap of the field magnet. This is because the air gap in the magnetic circuit which provides the confining magnetic field is considerably wider than the air gap in the core magnetic circuit which provides the bulk of the acceleration voltage, and because of the high permeability of the core material. On the one hand, with the magnetic induction (field strength) B at an electron-beam target as a reference, the excitation energy is approximately proportional to B.sup.2 ; on the other hand, the end-point energy of a relativistic beam in a betatron is proportional to r.multidot.B, where r is the radial position of the target. Furthermore, B is proportional to r.sup.-n, with n between 0 and 1. Thus, optimal power conversion is realized if the target is placed at the outer edge of the field magnet and if the electron beam is extracted to strike the target when B is at its peak. Since the beam must be located within r during acceleration, the beam radius has to be expanded as the magnetic field increases towards its peak.
Prior art techniques for beam extraction have reduced the confining magnetic field either with an extraction coil and switch arrangement in the field magnetic circuit, or with a magnetic flux clamping circuit in the core circuit. These techniques are relatively simple to implement, but neither effects extraction of the beam at its maximum possible energy.
Typically, in an accelerator-based logging tool, e.g., a density tool, the electromagnetic radiation is relatively intense and has a short duty cycle, and the detectors operate in an energy deposition mode. In the case of a betatron as the radiation source, this holds true at least for near-spaced detectors. However, the total radiation energy depends not only on the amount of charge accelerated per pulse (which affects radiation intensity but not spectrum shape), but also on the end-point energy, affecting spectrum shape. While variations in radiation intensity are scalable (e.g., doubling the intensity will double the detector count rate without regard to source-detector spacing) and thus are normalizable, variations in end-point energy affect the radiation transport processes and affect detector response differently at different spacings. It is important, therefore, that end-point energy variation be kept as small as possible.
In a betatron, one important factor that affects the extracted beam energy is extraction timing. Since the end-point beam energy is determined mainly by B (i.e., by the magnetic induction at the target), and since B varies essentially sinusoidally, it is desirable to extract the beam when B is at or near its peak, where a change in extraction timing has the least effect on beam energy.
Magnetic properties of materials change as a function of temperature. Where power consumption is of no concern, a sufficiently large air gap in a magnetic circuit will minimize the magnetic effects of temperature changes; this, however, is impractical in the case of a miniature betatron, for borehole use, for example. In the case of a cut core made of Metglas S-3 and SC and without a large gap, as described, for example, in the aforementioned U.S. Pat. No. 5,122,662, the apparent permeability drops with temperature, with the core current increasing by as much as 50 percent for a temperature change of 50 degrees C. A larger current results in a higher resistive loss in the core, and this in turn modifies the betatron condition in such a way that the electron orbit shrinks with temperature. In such circumstances, unless corrected, at the least extraction will be delayed, because the smaller the electron orbit before expansion, the longer it takes to expand it to the target. In the extreme case, electrons may even strike the inside of the betatron "donut" before expansion takes place. A change in the betatron condition may further result in excessive electron loss, and hence a reduction in beam intensity.